1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording medium, and more particularly to an image forming apparatus of so-called electrophotographic method for forming an image by forming an electrostatic latent image on the surface of a photosensitive body, depositing a developer on the electrostatic latent image and transferring it onto the recording medium.
2. Description of the Related Art
Conventionally, an image forming apparatus has been provided, including an photosensitive body, an optical scanning unit for deflecting and scanning a light on the photosensitive body to form an electrostatic latent image on the surface of the photosensitive body, a developing section for developing the electrostatic latent image by depositing a developer on the electrostatic latent image formed on the surface of the photosensitive body by the optical scanning unit, and a transfer section for transferring the developer deposited on the surface of the photosensitive body by the developing section to the recording medium. In the image forming apparatus of this type, the electrostatic latent image is formed on the surface of the photosensitive body by deflecting and scanning the light, and developed by the developing section depositing the developer on the electrostatic latent image. The transfer section transfers the developer deposited on the surface of the photosensitive body onto the recording medium, whereby the image according to the deflecting and scanning is formed on the recording medium by the so-called electrophotographic method.
The well known optical scanning unit forms an electrostatic latent image according to the image data on the surface of the photosensitive body by emitting and deflecting the light to a polygon mirror driven and rotated by a polygon motor, and by making the emitted light intermittent according to the image data.
In recent years, in such image forming apparatus, the rotation speed of the polygon mirror is increased along with the higher image formation speed. Therefore, there is possibility that the temperature within the optical scanning unit rises excessively. It is conceivable to cool the polygon motor by a fan. However, if the fan is provided, the size and cost of the apparatus are increased, with the great noise produced. Therefore, it is desired to make the apparatus silent and reduce the cost by eliminating the fan, if possible. It has been proposed that when a certain number of sheets are printed successively, for example, the sheet feed interval is increased to drive the polygon motor at low speed (e.g., refer to JP-A-2003-66812).